Math.easeInOutQuad = function (t, b, c, d) {
	t /= d / 2
	if (t < 1) {
		return (c / 2) * t * t + b
	}
	t--
	return (-c / 2) * (t * (t - 2) - 1) + b
}

// requestAnimationFrame 用于动画
var requestAnimFrame = (function () {
	return (
		window.requestAnimationFrame ||
		window.webkitRequestAnimationFrame ||
		window.mozRequestAnimationFrame ||
		function (callback) {
			window.setTimeout(callback, 1000 / 60)
		}
	)
})()

/**
 * @param {number} amount
 */
function move(amount) {
	document.documentElement.scrollTop = amount
	document.body.parentNode.scrollTop = amount
	document.body.scrollTop = amount
}

function position() {
	return (
		document.documentElement.scrollTop ||
		document.body.parentNode.scrollTop ||
		document.body.scrollTop
	)
}

/**
 * @param {number} to
 * @param {number} duration
 * @param {Function} callback
 */
export function scrollTo(to, duration, callback) {
	const start = position()
	const change = to - start
	const increment = 20
	let currentTime = 0
	duration = typeof duration === 'undefined' ? 500 : duration
	var animateScroll = function () {
		currentTime += increment
		var val = Math.easeInOutQuad(currentTime, start, change, duration)
		move(val)

		if (currentTime < duration) {
			requestAnimFrame(animateScroll)
		} else {
			if (callback && typeof callback === 'function') {
				callback()
			}
		}
	}
	animateScroll()
}
